English
Two submonoids are equal if they have the same elements.
Русский
Два подмономиода равны, если и только если у них одинаковые элементы.
LaTeX
$$$$ S = T \\iff (\\forall x, x \\in S \\leftrightarrow x \\in T) $$$$
Lean4
/-- Two submonoids are equal if they have the same elements. -/
@[to_additive (attr := ext) /-- Two `AddSubmonoid`s are equal if they have the same elements. -/
]
theorem ext {S T : Submonoid M} (h : ∀ x, x ∈ S ↔ x ∈ T) : S = T :=
SetLike.ext h